Time-resolved detection of relative intensity squeezed nanosecond pulses in a Rb87 vapor

We present theoretical and experimental results on the generation and detection of pulsed, relative-intensity squeezed light in a hot Rb87 vapor. The intensity noise correlations between a pulsed probe beam and its conjugate, generated through nearly-degenerate four-wave mixing in a double-lambda system, are studied numerically and measured experimentally via time-resolved balanced detection. We predict and observe about -1 dB of time-resolved relative intensity squeezing with 50 nanosecond pulses at 1 MHz repetition rate. (-1.34 dB corrected for loss).Comment: 11 pages, 9 figure

Ultrasonic wave propagation in reticulated foams saturated by different gases: High frequency limit of the classical models

International audienceTransmission experiments are performed on high porosity reticulated polyurethane foams saturated by different gases at ultrasonic frequencies up to 800 kHz. An excess attenuation is observed at high frequencies, when the wavelength is not sufficiently large compared to the lateral dimensions of the fibers. At lower frequencies, these experiments lead by using classical models of equivalent fluids, to a fast and reliable method for determining the characteristic length $\Lambda$

Study of Circumferential Waves on a Poroelastic Cylinder

International audienceThe dispersion relations of acoustic modes in poroelastic cylinders with and without elastic shell coating are determined and solved. The influence of elastic frame parameters and the Biot parameters on the dispersion curves is studied in the configuration with and without coating. The dispersive Rayleigh and whispering gallery waves are highly sensitive to the density and the shear modulus of the skeleton, opening aperspective for the evaluation of the mechanical parameters of poroelastic materials confined in cylindrical tubes during the manufacturing process. The predicted dispersion curves are validated with experimental results obtained by use of different experimental setup in the case of aporous circular cylinder

"Introduction" de l'ouvrage "De l'usage de l'art en politique"

Introduction de l'ouvrage "De l'usage de l'art en politique". L'usage de la pratique artistique dans une histoire culturelle du politique constitue un champ de recherche fécond que cet ouvrage se propose d'aborder. Les contributions envisagent la question dans l'Europe occidentale, du siècle des Lumières à la veille de la première guerre mondiale, c'est-à-dire pendant une longue période de bouleversements des systèmes et des représentations politiques. Acquisitions et dons d'œuvres d'art, construction d'une identité régionale ou nationale autour d'œuvres patrimoniales, représentation du pouvoir politique illustrent la variété des relations entre art et politique aux époques moderne et contemporaine. L'ouvrage ouvre une réflexion sur l'usage de l'art par le politique aujourd'hui

Measuring the dynamic shear modulus of poroelastic foams in the audible frequency range

International audienceThe prediction of acoustical properties of multilayered systems including poroelastic layers using the full Biot theory is in principle possible but in practice limited by the absence of material data. One of the parameters that is difficult to measure is the dynamic rigidity of the porous frame. Current experimental methods are limited to the lower part of the audible frequency range 1 (typically below 400 Hz) and require special shapes of the sample (cube, cylindrical rod or very thin samples). Since most sound absorbing plastic foams are viscoelastic, the elastic moduli may vary strongly with frequency, a measuring technique in the full audible frequency range is needed. Recently 2 a new method for the measurement of the dynamic shear modulus of the frame of poroelastic foams in the medium and high audible frequency range (1 to 4 kHz) has been presented This method is based on the measurement of the velocity and the damping of a Rayleigh-type surface wave on sample with thickness larger than the Rayleigh wave penetration depth. The Rayleigh wave was excited through direct mechanical excitation of the frame or the porous material and detected using a laser-doppler vibrometer. The velocity of this wave is closely related to the shear velocity, which is directly linked to the shear modulus. The damping of the Rayleigh wave can be used to determine the imaginary part of the shear modulus.In this work a first attempt is made to measure the dynamic shear modulus on a layer of finite thickness. In this way there is no requirement whatsoever concerning the shape of the sample under investigation

Guided elastic waves in porous materials saturated by air under Lamb conditions

International audienceThe propagation of guided elastic waves in porous materials saturated by air under Lamb conditions is studied theoretically and experimentally. The modes are derived from expressing the boundary conditions on the normal and tangential stresses and the displacements at the interfaces between the porous layer and the surrounding fluid. The stresses and the fluid pressure inside the porous medium are obtained from Biot's equations of poroelasticity. Symmetrical and antisymmetrical modes are found when the porous layer is loaded by the same fluid on both sides. Damping mechanisms include viscous and thermal exchanges between the solid and the fluid, in addition to the classical structural damping. Using an experimental setup based on the generation of standing waves in the layer and taking the spatial Fourier transform of the displacement profile, the phase velocities of three modes were measured for two porous materials in a frequency range between 80 Hz and 4 kHz. The measurements confirm the theoretical predictions and provide information on the shear modulus of a sound-absorbing material in a wide frequency range

Fecundity of cows and dairy herds in Wallonia. Management effects.

audience: professional, studentL’intervalle moyen entre vêlages est de 409 jours. Pour les 25 % meilleurs troupeaux (3125 annéesz troupeaux) et vaches (107.700 lactations), il est respectivement de 387 et 346 jours. L’objectif classiquement proposé d’un veau par vache et par an n’est atteint que dans 0,38% des troupeaux et par une vache sur quatre (27%). L’augmentation de la production laitière moyenne s’accompagne d’un allongement de l’intervalle entre vêlages. Des performances de reproduction comprises entre 380 jours pour les élevages dont la production est inférieure à 8000 kg et de 390 jours pour les élevages dont la production est supérieure à 8000 kg de lait sont néanmoins tout à fait possibles. L’impact de l’augmentation du niveau de production sur l’allongement de l’intervalle entre vêlages est limité. Les éleveurs dont les troupeaux ont des moyennes de production égales ou supérieures à 8.000 kg tirent un meilleur parti de l’environnement (effet positif) que ceux dont les troupeaux ont des moyennes de production inférieures à 7.000 Kg. La marge brute par vache augmente avec le niveau de production laitière moyen du troupeau